Gas Exchange: Humans

Question 1

Explain the essential features of the alveolar epithelium that make it
adapted as a surface for gas exchange:

Answer 1

1. Flattened cells / 1 cell thick → short diffusion distance
2. Folded → large surface area
3. Permeable → allows diffusion of O2 / CO2
4. Moist → gases can dissolve for diffusion
5. Good blood supply from large network of capillaries→
   maintains concentration gradient

Question 2

Describe how gas exchange occurs in the lungs:

Answer 2

1. Oxygen diffuses from alveolar air space into blood down its concentration gradient
2. Across alveolar epithelium then across capillary endothelium

Question 3

Explain the importance of ventilation:

Answer 3

1. Brings in air containing higher conc. of oxygen & removes air with lower conc. of oxygen
2. Maintaining concentration gradients

Question 4

Inspiration (breathing in):

Answer 4

1. Diaphragm muscles contract → flattens
2. External intercostal muscles contract, internal
   intercostal muscles relax (antagonistic) →
   ribcage pulled up / out
3. Increasing volume and decreasing pressure
   (below atmospheric) in thoracic cavity
4. Air moves into lungs down pressure gradient

Question 5

Expiration (breathing out):

Answer 5

1. Diaphragm relaxes → moves upwards
2. External intercostal muscles relax, internal
   intercostal muscles may contract → ribcage
   moves down / in
3. Decreasing volume and increasing pressure
   (above atmospheric) in thoracic cavity
4. Air moves out of lungs down pressure gradient

Question 6

Suggest why expiration is normally passive at rest:

Answer 6

1. Internal intercostal muscles do not normally need to contract
2. Expiration aided by elastic recoil in alveoli

Question 7

Suggest how different lung diseases reduce the rate of gas exchange:

Answer 7

1. Thickened alveolar tissue (eg. fibrosis) → increases diffusion distance
2. Alveolar wall breakdown → reduces surface area
3. Reduce lung elasticity → lungs expand / recoil less → reduces concentration gradients of O2 / CO2

Question 8

Suggest how different lung diseases affect ventilation:

Answer 8

1. Reduce lung elasticity (eg. fibrosis - build-up of scar tissue) → lungs expand / recoil less
   ○ Reducing volume of air in each breath (tidal volume)
   ○ Reducing maximum volume of air breathed out in one breath (forced vital capacity)
2. Narrow airways / reduce airflow in & out of lungs (eg. asthma - inflamed bronchi)
   ○ Reducing maximum volume of air breathed out in 1 second (forced expiratory volume)
3. Reduced rate of gas exchange → increased ventilation rate to compensate for reduced oxygen in blood

Question 9

Suggest why people with lung disease experience fatigue:

Answer 9

Cells receive less oxygen → rate of aerobic respiration reduced → less ATP made

Question 10

Suggest how you can analyse and interpret data to the effects of pollution,
smoking and other risk factors on the incidence of lung disease:

Answer 10

1. Describe overall trend → eg. positive / negative correlation between risk factor and incidence of disease
2. Manipulate data → eg. calculate percentage change
3. Interpret standard deviations → overlap suggests differences in means are likely to be due to chance
4. Use statistical tests → identify whether difference / correlation is significant or due to chance
   ○ Correlation coefficient → examining an association between 2 sets of data
   ○ Student’s t test → comparing means of 2 sets of data
   ○ Chi-squared test → for categorical data

Question 11

Suggest how you can evaluate the way in which experimental data led to
statutory restrictions on the sources of risk factors:

Answer 11

1. Analyse and interpret data as above and identify what does and doesn’t support statement
2. Evaluate method of collecting data
   ○ Sample size → large enough to be representative of population?
   ○ Participant diversity eg. age, sex, ethnicity and health status → representative of population?
   ○ Control groups → used to enable comparison?
   ○ Control variables eg. health, previous medications → valid?
   ○ Duration of study → long enough to show long-term effects?
3. Evaluate context → has a broad generalisation been made from

Question 12

Explain the difference between correlations and causal relationships:

Answer 12

1. Correlation = change in one variable reflected by a change in another - identified on a scatter diagram
2. Causation = change in one variable causes a change in another variable
3. Correlation does not mean causation → may be other factors involved